This invention relates generally to the field of receiving, storing and dispensing compressed natural gas, and more particularly relates to systems and methods for receiving natural gas from pipelines or delivery vehicles, compressing and storing the natural gas, and dispensing the natural gas into vehicles from fuel islands.
Using compressed natural gas (“CNG”) for motor vehicle fuel is relatively new in the U.S. It is anticipated that most CNG fuel islands, designed and structured to dispense CNG to individual vehicles in the manner of standard gas stations, will obtain natural gas by direct connection to utility pipelines, as provided by municipal infrastructure. However, the gas pressure and volume from these pipelines is insufficient to support fast-fill, i.e., CNG-on-demand, islands. Inconsistent demands for natural gas throughout the day by neighboring customers sharing the pipeline create widely disparate volume and pressures that are detrimental for optimum operation of open-traffic fast-fill CNG fuel islands directed at refueling motor vehicles. Decreased pipeline pressure increases the time required to fuel vehicles, and as more vehicles convert to CNG, the fuel demand may exceed the supply available to any given fuel island because the island is dependent on the supply offered by the neighborhood pipeline at any given time.
Furthermore, traditional fast fill CNG stations employ either 1) cascade storage systems or 2) buffer systems, or 3) a combination of the two. The problem with these traditional systems is that they are not conducive to multiple vehicle, high volume dispensing demands, such that multiple vehicles cannot pull up at a CNG station at the same time and get fueled with CNG as fast as a gasoline vehicle can refuel at a traditional gas station. The stumbling block is that the known CNG systems are limited by the throughput of the natural gas pipeline and compressor.
A cascade storage system is a high pressure gas cylinder system which is used for the refilling of smaller compressed gas cylinders. When gas contained in a cylinder at high pressure is allowed to flow to another cylinder containing gas at a lower pressure, the pressures will equalize to a value somewhere between the two initial pressures. Cascade systems typically employ 3 CNG tanks and a vehicle will first be filled from one of them, which will result in an incomplete fill, perhaps 2000 psi for a 3000 psi tank. The second and third tanks will bring the vehicle's tank closer to 3000 psi. The station typically has a compressor which refills the station's tanks using natural gas from the utility line. Each of the large cylinders is filled by a compressor, but the cascade system allows small cylinders to be filled without a compressor. This prevents accidentally overfilling the tank, which could happen with a system using a single fueling tank at a higher pressure than the target pressure for the vehicle.
The benefit of a cascade system is that it's a fast fill. But the system is only fast until pressure in the storage cylinders decreases to an ineffective dispensing level. The fill rate is then limited to the speed of the compressor, which could be a significant problem when trying to fill more than one vehicle at the same time. If the cascade system is overburdened the pressure in the storage cylinders decreases to an inefficient or inoperable level before the compressor can refill storage cylinders.
Buffer fast-fill stations primarily fuel directly from the compressor into the vehicle and therefore require a smaller quantity of storage. These stations typically serve a captive fleet and are designed and sized for the need and fueling patterns of the specific fleet. They are often located onsite and allow for large quantities of fuel to be dispensed in a relatively short period of time. The advantage of the buffered system is its fast fuel dispensing. However a disadvantage is that compressors are very expensive and the amount of gas available from the pipeline limits the dispensing output. In addition, utility costs to power the compressor can be very expensive if refueling is done at peak electricity times.
In a combination buffer/cascade system, the pipeline gas goes to the compressor and then fills up the buffered spherical tanks at, for example, 4200 psi, and these tanks are attached to the high side cascade tank, which fills the vehicle tank until it runs out of pressure, as stated above in the cascade system.
It is an object of this invention to address and solve the above problems by providing a system of CNG storage tanks and gas compressors which are designed to collect, store and compress natural gas on site in a manner that accounts for fluctuations in gas supply from pipelines or other sources and accounts for fluctuations in vehicle refueling demands. It is an object to provide such a system that is designed to maximize the speed and efficient dispensing of CNG into motor vehicles utilizing in combination a descending pressure storage bank and a high pressure storage bank.